An ignition device of a spark-ignition internal combustion engine is a device that generates discharge at a gap between electrodes of a spark plug to ignite fuel in an internal combustion engine.
A conventional ignition device performs ignition by generating spark discharge between electrodes of a spark plug with DC power generated by a DC power source, and then generating AC plasma between the electrodes of the spark plug with AC power generated by an AC power source. The conventional ignition device is characterized in that the AC power is reduced after AC plasma is generated between the electrodes. According to this conventional ignition device, it is possible to reduce the total energy supplied to electrodes by AC power for generating and maintaining AC plasma (see, for example, Patent Literature 1).